Lithium ion cells have become attractive for portable electronic devices such as cellular phones and laptop computers as they offer higher energy density than other rechargeable systems. Commercial lithium-ion cells currently have a lithiated carbon negative electrode, or anode, LixC6, and a lithium cobalt oxide positive electrode, or cathode, LiCoO2. During charge and discharge of the cell, Li ions are transported back and forth between the anode and cathode and intercalated into the host structures. The most common cathode material, LiCoO2, has a layered structure and operates at approximately 4 V vs. lithiated carbon. Unfortunately it is relatively expensive and is highly reactive in the oxidized or de-lithiated state, leading to safety concerns at high states of charge. The high reactivity also affects the life of the cell, the rate capability and generally prevents its use in cells larger than 18650 size, the conventional size of cell used in computer laptops.
Other materials are being developed as alternatives to LiCoO2. In particular the isostructural materials LiNiCoO2 and LiNiMnO2 have been proposed as alternatives. However, they also suffer from stability problems at high states of charge which limit their use in larger cells and in some cases have poor rate capability. More recently LiFePO4 has been developed as an alternative to these materials. While these materials are inherently safer because of their low voltage (3.5 V vs Li), these materials have inherently poor rate capability and low volumetric energy density when using conventional Li-ion cell building manufacturing processes. Engineering approaches have been used to improve their rate capability, though this leads to an even lower energy density system. Thus it is highly desirable to find new cathode materials that are both safe, high rate with reasonable energy density that can be to make larger Li-ion cells for the growing power tool, hybrid electric vehicle and stationary power markets.
The spinel electrode material LiMn2O4 is highly attractive for these applications because of its low cost, low toxicity and much greater safety. However, the LiMn2O4 electrode material tends to exhibit capacity fade in the Li-ion cell environment during cycling that is particularly severe above 45° C. A number of factors have been reported to be responsible for the capacity fade, many of which are related to the reactivity of the manganese spinel surface. For example, the dissolution of Mn2+ into the electrolyte has been reported to result from a disproportionation reaction of Mn3+ in contact with the electrolyte according to the reaction: 2Mn3+(solid)→Mn4+(solid)+Mn2+(solution).
Several attempts have been made to overcome the problems of capacity fade associated with LiMn2O4 materials. For example, cationic substitution for manganese changes the average oxidation state of the Mn ions to above 3.5, thus reducing the amount of Mn3+ ions in the fully discharged electrode. This approach has been shown to improve the capacity retention of the material at high temperatures. However, this approach also results in a significant decrease in the specific capacity of the spinel material. Other approaches have been taken such as protecting the material with a complete surface coating. For example, coating the material with a low temperature borate glass, with metal oxides/organics (Al2O3, MgO, YtO) or a coating of another Li-ion cathode active material such as LiCoO2 or LiNiCoO2. Although some success has been achieved, many of the approaches used are often expensive, difficult to control and difficult to implement at large volumes. The approaches also lead to large capacity loss or poor rate capability materials. There is still a need for further improvement in capacity retention in LiMn2O4 based lithium ion cells and for improvement of the state of the art protective coatings on these electrodes to improve the overall performance and safety of Li-ion cells.